In the machining operation of a workpiece by a cutting tool having a large number of edges (tips) as represented by a broach cutter tool, if even a single workpiece is damaged or suffers an abnormal wear, for example, when an abnormality is produced therein, the workpiece in the operation can no longer be machined satisfactorily or with a due precision since vibrations are generated therein and may result in an excessive load applied in other normal tips, bringing about a damage to or an abnormal wear of such other tips.
In order to eliminate such an inconvenience, various methods and means have been proposed in the art for detecting a damage to or abnormal wear of a cutting tip in the course of a machining operation.
For example, Japanese Unexamined Patent Publication No. Sho 61-111877 and No. Sho 62-74553 (Japanese Patent Publication No. Hei 1-33299) have disclosed a "tip damage detector" or a "cutting tool abnormality sensing method", designed to detect an abnormality in a cutting tool by sensing vibrations which are generated when a tip is damaged.
Also, Japanese Unexamined Patent Publication No. Sho 61-252051 proposed a "tip damage sensing apparatus" that is intended to optically detect tip damage in response to a signal sensed from a tip by a photoelectric switch having an optical fiber in combination with a signal derived from a tip mounting position sensing circuit.
Furthermore, Japanese Unexamined Patent Publication No. Hei 6-39685 has proposed a "cutting tool damage sensing apparatus" which is designed to detect wear of or damage to a cutting tool using an image pickup means such an optical sensor or a TV camera.
An abnormality sensing method or apparatus of the type disclosed in Japanese Patent Publication No. Sho 61-111877 and No. Sho 62-74553 mentioned above, in which the detection of vibrations caused by damage to or abnormal wear of a tip have been found to be inconvenient when used in a finish machining operation where a cutting rate is low and thus tool damage or an abnormality is seldom followed by vibrations as described and hence is hard to be detected.
A difficulty in detecting a tool abnormality is especially acute in a turn broach machine (herein simply referred to "turn broach"). The turn broach uses a cutting tool (herein referred to as "broach cutter"), having tool tips mounted in a plurality of rows, where vibrations caused by a tip abnormality are superimposed one upon another. Hence, such a tool abnormality is hardly identifiable with respect to which particular tip upon which the vibration has occurred.
As a result, when a tool abnormality is identified, the particular tip containing the abnormality, of as much as several hundreds tips, must be located without requiring a large number of steps.
On the other hand, an abnormality sensing apparatus of the type disclosed in Japanese Unexamined Patent Publication No. Sho 61-25205 and No. Hei 6-39685 and designed to detect a tip abnormality by a photoelectric switch with an optical fiber or an image pickup means has been found to be inconvenient in that a tip abnormality can only be detected with difficulty if the tip is fine so as to be detected with an unsatisfactory degree of precision. When the surface of a tip used to produce a light reflection to be detected is thermally oxidized by heat generated during a cutting operation and is thereby blackened, its reflectivity will be sharply reduced so that sensing its abnormality may no longer be possible.
Furthermore, an image pickup means in the prior art system has been found to be impractical when used to sense an abnormality of a broach cutter in a turn broach having tool tips varied in position diametrically with respect to its center of rotation where the image pickup means cannot be kept focused on a workpiece cutting tool tip as the broach cutter is rotated causing the image pickup means to be variably distant from the tool tip and thus rendering the image pickup means defocused even if the image pickup means is focused on the workpiece cutting tool tip firstly.
It is accordingly an object of the present invention to eliminate these conventional difficulties and to provide a turn broach abnormality sensing apparatus of a non-contact type in a broach cutter in which a plurality of tips are arranged eccentrically with their eccentricity varying in succession.